The present invention relates to nondestructive continuous testing of tubes or other hollow sections having longitudinal or spiral welding seams.
Tubing, hollow sections, or other profiles are usually tested as to internal or external flaws and defects, particularly in the region of the seam. Such tests are conducted in a variety of ways; among them is the measurement of the magnetic stray flux and/or of eddy current components, etc. which are modified by any such defect. In particular, the pipes or tubes are scanned from the outside by means of magnetic probes e.g. by a.c. biased coils, and the interaction thereof with the pipe or tube material produces an output from which one extracts indications as to any flaws or absence thereof. It was found, however, that the sensitivity of this method is rather low for defects in the interior, i.e. adjacent the inner wall surface of the tube and of the seam. This drawback can be remedied either by applying a magnetic test field also along the inner surface of the tube. However, this additional step introduces an aspect of discontinuity into the process. Alternatively, the scanning field applied from the outside can be increased to such an extent that it penetrates sufficiently deep into the pipe's material and particularly into the seam. That method, however, has the drawback that actually the strong a.c. field as applied, causes the pipe's material to be heated very strongly so that a cooling step must follow. Instead of a.c. test and measuring currents, one can use d.c. to avoid parasitic heating. Flaws and defects modify also the magnetization induced by a d.c. biased coil which moves relative to the pipe. However, the pipe must be demagnetized subsequently. Moreover, the relative movement between test object and test equipment must be rather accurately controlled.